what's wrong with this?

<%= render :partial => 'tags/form',
                  :locals => {:form => post_form} %>

Missing partial tags/form with
{:handlers=>[:erb, :coffee, :builder], :formats=>[:html], :locale=>[:en]}.

is there a partial in this format: "_form.html.erb"? or do you want to
access post_form in the folder "tags"?

‘tags/form’ is a relative path, you should ‘/tags/form’ to absolute path.